1. Field of the Invention
The present invention relates to a pile head dresser which is mounted to the head of a steel cage of a cast-in-place concrete pile installed in the ground for making a foundation for civil engineering work, to a method of dressing the head of such a concrete pile, and to an improvement in placement of such a pile.
2. Description of Related Art
In common construction work, a variety of methods are used for driving and installing foundation piles. Among of them is a popular cast-in-place pile installation method.
The cast-in-place concrete pile installation method comprises drilling a round hole into a bearing base layer in the ground by an earth drilling technique, installing in the drilled round hole a steel cage consisting of a framework assembly of reinforcement bars, and filling the hole with a batch of concrete which is supplied through a tremie pipe having been inserted from above into the steel cage. A reinforced concrete pile is formed when the concrete is solidified.
In general, the batch of concrete is abundant thus having an extension of about 1 meter high above the steel cage of the concrete pile. The concrete pile with the extension is covered with refilling soil for curing.
After the curing of the concrete is completed, the soil is removed and the concrete extension is chipped off to expose the reinforcement bars at the top of the steel cage. The foundation of a building structure is then built on the concrete piles.
The chipping of the concrete extensions however creates unwanted sounds, vibrations and dust and is generally regarded as a public nuisance. Also, it may damage the reinforcement bars and cause labor accidents and industrial illnesses. The concrete extensions require a considerable amount of concrete and the chipping procedure increases the overall construction cost and time.
A method of eliminating the chipping procedure is disclosed in Japanese Patent Application 61-186616 (1986) where the concrete extensions are removed while the concrete, supplied through a tremie pipe, remains in a viscous fluid form before being solidified. The removal of the concrete is achieved by vacuum suction with a suction hose coupled to a heavy-duty vacuum pump. The method is only feasible with the use of a casing installed in the drilled hole and is not satisfactorily applicable to any different case where the drilled hole may easily collapse.
According to the method, the reinforcement bars are exposed and directly covered by refilling soil when the concrete extensions have been removed. Also, the removal of the concrete extensions requires a large machine which may easily damage the top of the exposed reinforcement bars. As the injured reinforcement bars have to be repaired with extra care and cost, the method is rather impractical.
To prevent such damage to the exposed reinforcement bars during the chipping or removal of concrete extensions, a reinforcement bar cap structure has been introduced as disclosed in Japanese Patent Application 58-153816 (1983).
The cap has a through hole provided in the center thereof for accepting and detachably coupling a tremie pipe. The cap comprises a closed double-wall structure of a doughnut shape having two separate rooms or spaces about the center hole which are defined by two, top and bottom, annular plates provided with apertures.
In use, joint bolts are inserted through the apertures of the top or bottom plate of the cap and threaded into sleeve joints of which rear ends are mounted to the uppermost ends of reinforcement bars of a steel cage so that the cap structure and the steel cage are tightly joined to each other. This is followed by introduction of a flow of concrete through a tremie pipe.
The flow of concrete into the steel cage is continued until the cap structure on the top of the steel cage is filled up. For curing the concrete in the steel cage, the steel cage and the cap are covered with refilling soil. When the concrete has been cured, the refilling soil is removed to expose the cap. The concrete in the cap structure is chipped off and retaining screws are removed so as to unlock the cap. After the joint bolts are separated from their respective sleeve joints, the cap structure is removed. Anchor bars are then threaded into the sleeve joints so that they extend upright from the reinforcement bars.
With the use of such a cap structure, the extensions of concrete piles are decreased in size but still require a chipping process for removal. The cap structure is not simple as it includes two parts which are joined to each other using side plates in an extra step of an assembling procedure. The anchor bars are threaded into their respective sleeve joints in order to increase a total length of the reinforcement bars. This is a rather delicate operation because it is hard to verify the mechanical strength thereof and it depends primarily on experimental skills. If the mechanical strength is worse, the means may cause critical accidents. It is said that the means is still being refined in order to overcome all of the technical drawbacks.
Some modifications of such a cap structure or a pile head dresser (the former being used hereinafter) for use in installing bearing piles have been developed, including one which is invented by he present inventors and is disclosed in Japanese Patent Application 1-322020 (1989).
The cap structure invented previously by the present inventors, unlike the conventional cap of a doughnut shape having two separate spaces and disclosed in the previously described application 58-152816, comprises an inner cylinder (tremie pipe introducing tube) arranged open at upper and lower ends for freely accepting a tremie pipe, and an outer cylinder (cap upper) disposed outwardly of the inner cylinder for coupling top and bottom plates, thus forming a simple double-cylinder structure. Also, the bottom plate has a plurality of reinforcement bar apertures provided in an outward region thereof (cutting projected face dish). The terms in parenthesis represent the counterpart structures in the previous Application.
Our previous cap structure of a cylindrical shape has a size substantially equal to the concrete extension and greater in depth than the conventional cap (structure) shown in Application 58-153816. In use, the cap structure is fitted onto the reinforcement bars so that its top plate comes into direct contact with the top of the reinforcement bars. Then, a tremie pipe is inserted into the inner cylinder for placement of concrete without forming a concrete extension. For curing the concrete, the cap structure is covered with refilling soil.
After the curing is completed, the refilling soil is removed to expose the cap structure. Since chipping work is not necessary, the reinforcement bars are exposed when the cap structure has carefully been removed. It is only necessary to remove the remaining concrete slime from the bottom of the inner cylinder while care is taken so as not to injure the reinforcement bars. After other unwanted deposition and slurry are removed, the concrete placement is finished.
It is well known that a conventional method of concrete placement comprises the steps, shown in FIG. 19(A) to 19(G), of placing a surface protective casing 7B and a steel cage 7C in a drilled hole 7A, removing a sediment of slime 7D from the bottom by means of an air lift or suction pump, and filling the drilled hole with a batch of concrete 7F supplied through a tremie pipe 7E. Also, an extension of concrete 7G is added to the pile concrete 7F for a predetermined height during the filling step. The concrete extension 7G contains impurities of slime and laitance and has to be chipped off after solidified.
The step of removing the slime sediment from the bottom is necessary for avoiding a reduction in the quality of a finished concrete pile caused by residual slime. It may be executed by suction with a known air lift or suction pump. Simultaneously, it is necessary to replace the slime sediment with a fresh supply of water for preventing lowering of a water level which may cause collapse of the inner wall. This will disturb the efficiency in carrying out the step. Technically, the removal of the slime sediment is very difficult and the concrete extension 7G is provided for compensation.
Our previous cap structure disclosed in the Application 1-322020 is designed for protecting the head of a pile as well as eliminating forming and chipping of the concrete extension which are essential in the prior art. Accordingly, as the chipping process is eliminated, no environmental problems will occur. Also, the overall procedure will be simplified and the consumption of concrete will be minimized, thus reducing the overall costs.
It is still troublesome to remove the remaining concrete slime from the inner cylinder when the concrete has been placed, covered, and solidified. Although the removal of the remaining concrete slime is carried out by in simple manner such as drilling the concrete along a cut line, splitting it with the use of wedge laggings or expanders, and lifting up the split concrete slime with ropes, it may preferably be omitted.
The bottom of the previous cap structure is flat and will be likely to accumulate and hold sediments of concrete slime or impurities. It is also essential to allow a liquid stabilizer used for preventing collapse of the inner wall and a water inflow welling up so as to pass smoothly through the cap structure and between the cap structure and the casing while exerting as small a pressure as possible on the cap structure. It is thus an object of the present invention to solve the foregoing problems attributed to the previous cap structure of a closed form having only reinforcement bar apertures.
It is another object of the present invention to provide an improved concrete placement method which avoids the procedure of removing sediments of concrete slime and allows a minimum of slime to enter the concrete.